Focused piezoelectric transducer and method of making

ABSTRACT

An apparatus and method for fabricating focused high frequency electromechanical transducers especially useful in ultrasonic inspection apparatus is disclosed. An initially flat or planar piezoelectric crystal is placed between a pair of members having mating surfaces contoured to correspond to the desired shape of the crystal. When the members are forced together the crystal is bent or otherwise formed into the desired shape.

United States Patent McElroy 51 May 30,1972

[ FOCUSED PIEZOELECTRIC TRANSDUCER AND METHOD OF MAKING [72] Inventor:Jerry T. McElroy, Boulder, Colo.

[73] Assignee: Automation industries, lnc., Century City,

Los Angeles County, Calif.

[22] Filed: June 17,1970

[21] App]. No.: 46,897

52 u.s.c| ..310/9.6,73167.7,73/715,

310/94, 310 95, 29/2535 51 Int. Cl. ..H04r 17/00,G0ln 29/04 58FieldofSearch ..73/7l.5,67.8;310/8.7,9.6,

[56] References Cited UNITED STATES PATENTS 3,233,449 2/1966 Han-non..73/67.8 2,803,129 8/1957 Bradfield. ..73/67.8

3,242,552 3/1966 Cowan ..29/25.35

Primary lixaminer--Richard C. Queisser Assistant Examiner-John P.Beauchamp AttorneyDan R. Sadler [5 7] ABSTRACT An apparatus and methodfor fabricating focused high frequency electromechanical transducersespecially useful in ultrasonic inspection apparatus is disclosed. Aninitially flat or planar piezoelectric crystal is placed between a pairof members having mating surfaces contoured to correspond to the desiredshape of the crystal. When the members are forced together the crystalis bent or otherwise formed into the desired shape.

1 1 Claims, 4 Drawing Figures Patented May 30, 1972 3,666,979

2 Sheets$heet 1 Fig.1

Jerry T. McElroy,

INVENTOR.

ATTORNEY.

Patented May 30, 1972 3,666,979

2 Sheets-Sheet 2 Pulse Deflection J Generator Generator Transmitter I Ri Deflection Generator l 62 el I H Fig 4.

Jerry T. McElroy,

INVENTOR.

ATTORNEY FOCUSED PIEZOELECTRIC TRANSDUCER AND METHOD OF MAKINGBACKGROUND This invention relates to electromechanical transducers, thefabrication and use thereof, and, more particularly, to new and improvedultrasonic transducers for transmitting and/or receiving focusedultrasonic energy having high frequencies.

In one form of nondestructive testing, ultrasonic energy is transmittedinto and/or received from a workpiece. By determining .the interreactionof the ultrasonic energy with the workpiece the physical characteristicsof the workpiece can be determined. In the so-called pulse echo form ofultrasonic testing a short, high frequency, electrical pulse is appliedto a transducer. The electrical pulse causes the transducer to vibrateand radiate a pulse of ultrasonic frequency. Conversely ultrasonicenergy incident upon the transducer results in the generation of anelectrical signal.

The ultrasonic energy is typically coupled to a workpiece to be testedthrough a suitable intervening couplant. The ultrasonic energy travelsthrough the workpiece under test and is redirected by the surfacesthereof as well as by any discontinuities which may be present in theworkpiece. The redirected energy may be received by the original ortransmitting transducer or a separate receiving transducer.

The resultant electrical signal from the transducer is coupled tosuitable utilizing means for indicating the characteristics of theworkpiece. For example, the electrical signals may be coupled to adisplay device such as a cathode ray tube. The resultant display mayprovide a visual indication of the depth, position, size, location, etc.of a discontinuity.

In order to improve the resolution accuracy, etc. of an ultrasonicnondestructive testing system it is highly desirable to employultrasonic energy of a very high frequency. The frequency at which thecrystal transducer vibrates is a function of its physical dimensions. Asa result, as the frequency of the ultrasonic energy increases thethickness of crystal decreases. Since piezoelectric materials are verybrittle and frangible, when the frequency is in a region above a fewmegacycles the crystal is so thin it become very delicate and fragile.This in turn makes the crystal extremely difficult to handle,particularly during manufacturing. Also to improve the resolution andaccuracy it is highly desirable for the crystal to only move or vibratein accordance with the applied electrical signal. However, because ofthe inherent characteristics of the crystal even though the electricalsignal may comprise only a unipolar half cycle" the crystal will ring orcontinue to vibrate for a considerable period of time after thetermination of the electrical signal. In order to overcome thesedifficulties it has been customary to apply some form of mechanicaldampening to the backside of the crystal.

To further improve the resolution and accuracy it is desirable to focusthe ultrasonic energy into a beam having preselected characteristics.One means of accomplishing this is to use acoustic lenses in front ofthe transducers. Although acoustic lenses are capable of focusing thebeam, they have not proven satisfactory at the higher frequencies forseveral reasons, for example the higher frequency filtering affects ofsuch lenses, etc.

Another means of obtaining a focused beam of ultrasonic energy is toeither cast or grind the piezoelectric crystal into the desired shape.However, this has also proven unsuccessful. This is particularly truewith crystal transducers intended for use at high frequency applicationswhere the crystal must inherently be very thin.

SUMMARY The present invention provides means for overcoming theforegoing difficulties. More particularly the present invention providesa novel ultrasonic transducer particularly useful in ultrasonic testingapparatus and the like and a method and apparatus for fabricating highfrequency, focused transducers.

In the embodiment of the invention disclosed herein, means are providedfor forming a transducer such as a piezoelectric crystal into a shapewhich is inherently effective to radiate a focused beam of ultrasonicenergy and is inherently self dampened. This is accomplished by forming(for example by lapping, etc.) a crystal blank into a thin, flat sectionsuitable for a crystal transducer to be used at the intended frequency.The thin crystal transducer is positioned between a pair of membershaving surfaces thereon contoured to correspond to the final desiredshape. The two members are then forced together so as to cause thecrystal to be bent or otherwise formed into the desired shape. Ifdesired, one of the members may be a permanent backing member. In thisevent an adhe sive or bonding material is provided on the contouredsurface and/or the crystal transducer whereby the backside of thetransducer is permanently retained in intimate engagement with themember.

It has been found that even though the crystal transducer may be verythin and otherwise quite fragile the compensating stresses introducedinto the crystal transducer during its forming prevents it fromdisintergrating. In addition, the resultant stressed configuration ofthe crystal transducer apparently is effective to materially improve the0" of the crystal transducer and thereby insure a high degree ofoscillatory dampenmg.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of anapparatus embodying one form of the present invention and particularlyadapted for fabricating improved ultrasonic crystal transducers;

FIG. 2 is a diagrammatic view of an apparatus which may be employed topractice a further advanced step in the production of the improvedcrystal transducers;

FIG. 3 is a view of an ultrasonic search unit employing a crystaltransducer embodying one form of the present invention; and

FIG. 4 is a block diagram of an ultrasonic test system incorporating theimproved transducer of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to the drawings in moredetail the present invention is particularly adapted to be embodied in anovel crystal transducer and a method for forming the same. Initially acrystal blank 10 of a polarizable, piezoelectric material, such as leadzirconate or barium titanate is provided. A pair of parallel surfaces 11and 12 are formed perpendicular to the X axis. Normally the thickness ofthe blank 10, i.e. the space between the surfaces 11 and 12 isconsiderably greater than the thickness of the finished crystaltransducer.

At this point it is desirable to polarize the material in the blank 10for the desired mode of vibration. Although the polarization may beaccomplished in any desired manner in the present instance a pair ofelectrodes 13 and 14 are applied to the two surfaces 13 and 14. Theelectrodes 13 and 14 are normally relatively thin layers of silver.

A source of polarizing potential such as a battery 17 and potentiometer18 are coupled to the electrodes 13 and 14 by the electrical connectors15 and 16. The blank 10 is now polarized by the application of asuitable potential from sources such as a battery 17 and potentiometer18. The blank 10 is preferably maintained at an elevated temperature ina region of about 400C during at least a portion of the polarizationcycle.

After the blank 10 is adequately polarized the temperature of the blank10 is appropriately reduced and the connections 15 and 16 are removed.

Although the blank 10 may be polarized in any desired manner for anydesired mode of operation in the present instance it is polarized alongthe X" axis for operation in the thickness-expander mode.

After polarization, the thickness of the piezoelectric blank 10 may bereduced down to the desired amount by any wellknown technique such aslapping.

The lapping operation may be continued until the thickness of the blankis reduced to the final thickness of the crystal transducer 37. Thisfinal thickness usually depends upon the operating frequency at whichthe crystal transducer 37 is intended to operate. For example if thetransducer 37 is intended to operate in the region of 20 mos it will beprovided with a thickness of about 0.005 inches.

The lapping operation inherently removes at least one of the electrodes.The other electrode may be left on the unlapped surface. After thecrystal transducer 37 is reduced to the desired thickness it may or maynotbe desirable to apply a second electrode 14 to the just lappedsurface 12'. The crystal transducer 37 is then positioned upon a formingmember 35 having a contoured surface. The contour of the surface isselected to correspond to a shape of the crystal transducer 37 whichwill produce the desired focusing, etc. Although this contour may beconcave or convex, cylindrical, parabolic, etc. in the present instanceit is concave and substantially spherical.

The end of a ram 30 is positioned adjacent the contoured surface on themember 35. The end of the ram 30 has a surface 31 which is contoured tocompliment the surface of the member 35. The ram 30 is adapted to have aforce applied thereto whereby it will be advanced in the direction ofthe arrow 20.

The end surface 31 is effective to engage the electrode 13 and/or thesurface 11 of the crystal transducer 37. As the ram advances in thedirection of the arrow 20 the thin crystal transducer 37 is forced intothe cavity on the end of the member 35.

Piezoelectric materials which are normally employed for ultrasonictransducers (for example materials such as lead zirconate, bariumtitanate, etc.) are very brittle, frangible, etc. As a result thesecrystals are normally considered to be very delicate, requiring verycareful handling in order to avoid breakage. However, in spite of thesecharacteristics of the crystal transducers, it is possible to bend orotherwise deform the transducer 37 over a sufiicient range to obtain aconsiderable amount of focusing. By way of example, it has been foundthe crystal transducer 37 may be formed into a spherical shape with aminimum amount of breakage when the radius of the sphere is at least sixtimes greater than the diameter of the transducer 37.

After the crystal transducer 37 is formed into the desired shape it maybe secured to a suitable backing member. The crystal transducer 37 maybe initially shaped upon a separate forming member and then secured to asecond member. However, it has been found advantageous for the formingmember 35 to be a permanent backing member. In this event a film 38 of abonding material such as an epoxy adhesive may be provided on thesurface of the backing member 35 either during the forming of thecrystal transducer 37 or as a subsequent operation. At any rate whenthis film 38 has cured the crystal transducer 37 is securely andpermanently bonded to the backing member 35.

The backing member is preferably fabricated from a suitable acousticdampening material. This is effective to absorb the acoustic energyradiated and/or reflected, etc. from the back surface 12. This backingmember may be similar to that disclosed and claimed in U.S. Pat. No.2,972,068.

It has also been found desirable for the backing member 35 to beelectrically conductive. By way of example, it may be a casting epoxyresin which is at least partially loaded with lead, tungsten, etc. Inthis event the bonding material 38 is electrically conductive.

It is to be noted that when the crystal transducer 37 is deformed andbonded in this deformed condition there will be a large amount ofinternal stress in the transducer 37. These stresses are believed to beeffective in greatly improving the Q" of the crystal transducer 37. Moreparticularly, it is be lieved these internal stresses materiallyinterfere with the physical movements of the crystal and cause thevibrations therein to be rapidly dissipated. This means that if a signalis applied to the crystal transducer 37 whereby it is made to vibrate itwill rapidly return to its quiescent state in an extremely shortinterval of time.

A pair of electrical leads 40 and 41, FIG. 3, may be connected to theelectrode 13 and the electrically conductive backing member 35 or theelectrode 14' if it is present. This entire arrangement may then besealed into a suitable search unit 60. v

The search unit 60 may be incorporated into an ultrasonic test systemsuch as shown in FIG. 4 for inspecting materials such as a workpiece 70.A pulse generator 50 provides high frequency signals to a transmitter52. The pulse generator 50 normally determines the pulse repetition rateof signals produced by the transmitter 52. The signals from thetransmitter 52 are applied to the electrodes 13 and 14' of the crystaltransducer 37 in search unit 60. The transducer 37 is physicallydeformed in response to the electrical signals and radiatescorresponding ultrasonic energy. Since the surface of the transducer 37is contoured the energy is focused into a beam of the desired shape.

The ultrasonic energy in the beam 80 is incident upon the workpiece andpropagates into and/or through the workpiece. When the energy isincident upon a discontinuity such as one of the surfaces of workpiece70 or a defect 71 therein at least a portion of the energy is returnedto the search unit 60 and/or to a second search unit 61.

When the returning ultrasonic energy is incident upon the crystaltransducer 37 or the transducer in the search unit 61 a correspondingelectrical signal is generated. The electrical output signals fromsearch unit 61 are applied to a receiver 53. It should be noted thatalthough two search units are shown the crystal transducer 37 isreciprocal and may be used as a transmitter and/or a receiver. Thesignals from receiver 53 are then applied to a deflection generator 54which applies them to the vertical deflection plates 56 and 58 of acathode ray tube 55, for example.

The horizontal time base for the cathode ray tube 55 is provided by adeflection generator 51 which is initiated by the pulse generator 50upon generation of the initial impulses. The deflection signal isapplied to the horizontal deflection plates 59 and 57 on the cathode raytube 55. The cathode ray tube, therefore, shows vertical deflections ona horizontal time base on the display 80.

What is claimed is:

l. A method of forming a focused ultrasonic transducer including thesteps of:

positioning a ceramic piezoelectric element that is in solid form upon asurface having a contour corresponding to the desired final shape of thetransducer;

forming said element while it is in said solid form around said surfaceinto said final shape; and

retaining said element in said shape.

2. The method of claim 1 wherein the steps of forming said element intosaid shape creates a stress state in said element and retaining saidelement in said shape includes bonding said element onto a backingmember and maintaining said element in the stressed state.

3. A method of forming a focused electromechanical transducer includingthe steps of:

providing a ceramic piezoelectric blank having a pair of parallel flatsurfaces;

polarizing said piezoelectric blank;

lapping at least one of said surfaces to reduce the thickness of theblank to the desired thickness of the transducer;

positioning the lapped blank on a mounting member having I a surfacewith a contour corresponding to the desired contour of the transducer;applying an adhesive material between the surface of the mounting memberand the blank; and pressing the blank against the surface of themounting member and bonding the blank to the mounting member. 4. Themethod of claim 3 wherein the step of pressing the blank against the topsurface of the mounting member includes the steps of:

positioning a ram member, having a surface corresponding to the-surfaceon the mounting member in engagement with the blank; and

forcing the surface of the ram against the blank whereby the blank isforced against the surface of the mounting member.

5. The process of forming a focused piezoelectric crystal by positioninga substantially flat thin ceramic piezoelectric crystal, having firstand second spaced parallel surfaces, on a mounting member having acontoured mounting surface adjacent said second crystal surface;

applying an adhesive material into the area between said second crystalsurface and said mounting surface; and

applying pressure to said first crystal surface to force said secondcrystal surface into intimate engagement with said mounting surfaces,said pressure being applied for a suffrcient time to allow said adhesiveto hold said crystal in the bent condition.

6. The process of claim 5 wherein the last named step comprises thesteps of:

positioning a ram member, having a convex surface substantially matchingthe radius of curvature of the concave holding member surface, adjacentsaid first crystal surface; and

forceably engaging said first crystal surface with the convex ramsurface to force said second crystal surface into intimate engagementwith the concave mounting surface.

7. A focused electromechanical transducer comprising:

a mounting member having a curved mounting surface;

a thin ceramic piezoelectric crystal having first and second spacedsubstantially parallel surfaces with the second surface being inintimate engagement with said curved mounting surface, said firstcrystal surface being in a state of compression and said second crystalsurface being in a state of tension; and

means for holding said second crystal surface in intimate engagementwith said curved mounting surface.

8. The transducer of claim 7 wherein said stressed piezoelectric crystalis spherical in configuration having a center of curvature substantiallydefining a point.

9. The transducer of claim 7 wherein said stressed piezoelectric crystalis cylindrical in configuration having a center of curvaturesubstantially defining a line.

10. The transducer of claim 7 wherein said holding means comprises acement material interposed between said mounting surface and said secondcrystal surface.

11. The transducer of claim 10 further comprising a thin conductivelayer positioned on said first crystal surface.

1. A method of forming a focused ultrasonic transducer including thesteps of: positioning a ceramic piezoelectric element that is in solidform upon a surface having a contour corresponding to the desired finalshape of the transducer; forming said element while it is in said solidform around said surface into said final shape; and retainiNg saidelement in said shape.
 2. The method of claim 1 wherein the steps offorming said element into said shape creates a stress state in saidelement and retaining said element in said shape includes bonding saidelement onto a backing member and maintaining said element in thestressed state.
 3. A method of forming a focused electromechanicaltransducer including the steps of: providing a ceramic piezoelectricblank having a pair of parallel flat surfaces; polarizing saidpiezoelectric blank; lapping at least one of said surfaces to reduce thethickness of the blank to the desired thickness of the transducer;positioning the lapped blank on a mounting member having a surface witha contour corresponding to the desired contour of the transducer;applying an adhesive material between the surface of the mounting memberand the blank; and pressing the blank against the surface of themounting member and bonding the blank to the mounting member.
 4. Themethod of claim 3 wherein the step of pressing the blank against the topsurface of the mounting member includes the steps of: positioning a rammember, having a surface corresponding to the surface on the mountingmember in engagement with the blank; and forcing the surface of the ramagainst the blank whereby the blank is forced against the surface of themounting member.
 5. The process of forming a focused piezoelectriccrystal by positioning a substantially flat thin ceramic piezoelectriccrystal, having first and second spaced parallel surfaces, on a mountingmember having a contoured mounting surface adjacent said second crystalsurface; applying an adhesive material into the area between said secondcrystal surface and said mounting surface; and applying pressure to saidfirst crystal surface to force said second crystal surface into intimateengagement with said mounting surfaces, said pressure being applied fora sufficient time to allow said adhesive to hold said crystal in thebent condition.
 6. The process of claim 5 wherein the last named stepcomprises the steps of: positioning a ram member, having a convexsurface substantially matching the radius of curvature of the concaveholding member surface, adjacent said first crystal surface; andforceably engaging said first crystal surface with the convex ramsurface to force said second crystal surface into intimate engagementwith the concave mounting surface.
 7. A focused electromechanicaltransducer comprising: a mounting member having a curved mountingsurface; a thin ceramic piezoelectric crystal having first and secondspaced substantially parallel surfaces with the second surface being inintimate engagement with said curved mounting surface, said firstcrystal surface being in a state of compression and said second crystalsurface being in a state of tension; and means for holding said secondcrystal surface in intimate engagement with said curved mountingsurface.
 8. The transducer of claim 7 wherein said stressedpiezoelectric crystal is spherical in configuration having a center ofcurvature substantially defining a point.
 9. The transducer of claim 7wherein said stressed piezoelectric crystal is cylindrical inconfiguration having a center of curvature substantially defining aline.
 10. The transducer of claim 7 wherein said holding means comprisesa cement material interposed between said mounting surface and saidsecond crystal surface.
 11. The transducer of claim 10 furthercomprising a thin conductive layer positioned on said first crystalsurface.